The widely used M109 carries a 155-mm howitzer and is the principal self-propelled artillery support for U.S. Army divisions. It is a large tracked vehicle with a fully traversable turret and prominent bustle. Early variants had a short, 23-caliber barrel. Later versions, including the M109A6 Paladin, have a 39-caliber barrel.

The M109 has a crew of six: commander, gunner, driver and three ammunition members. The hull is made of all-welded aluminum armor, which protects against small-arms fire and shell splinters.

The driver is seated at the front left of the hull, with the powerpack to the right and the turret at the rear. The driver is provided with a single-piece hatch cover that opens to the left, with three M45 day periscopes in front. These can be covered with small metal flaps to prevent damage. One of the day periscopes can be replaced by a passive night-vision periscope.

The all-welded aluminum armor turret at the rear of the hull has a square hatch in each side that opens to the rear and twin doors in the turret rear. The commander is seated on the right side of the turret and has a cupola that can be traversed through 360 deg, a single-piece hatch cover that opens to the rear and an M27 day periscope. A .50-cal M2 heavy-barrel machine gun is pintle-mounted on the front of the cupola.

The gunner is seated on the left side of the turret and has a square single-piece hatch cover that opens to the right. The twin doors at the rear of the turret are provided for ammunition resupply. Mounted at the rear of the hull on each side of the hull door is a large spade that is manually lowered to the ground before firing. They are normally deployed only when firing top charges.

The torsion bar suspension on either side consists of seven dual rubber-tired road wheels with the drive sprocket at the front and the idler at the rear. There are no track-return rollers. The tracks are of a single-pin, center-guide type with replaceable rubber pads.

The M109 is fitted with night-vision equipment, but does not have nuclear, biological and chemical (NBC) warfare protection. The vehicle can be fitted with an amphibious kit consisting of nine air bags, four on each side of the hull and one at the front. The bags are inflated from the vehicle, which can then propel itself across rivers on its tracks at a maximum speed of 4 mph (6 kmh).

The main armament is a 155-mm M126 howitzer in an M127 mount with a fume extractor and large muzzle brake. The recoil system is hydropneumatic and the breech block is of the Welin-step thread type. Gun elevation and depression and turret traverse are hydraulic with manual controls for emergency use.

Fire-control equipment includes an elbow telescope M118C for direct fire with a magnification of 4x and 10-deg field of view; panoramic telescope M117 for indirect fire and gunner's quadrant M1A1 .

The M109 Paladin self-propelled Howitzer, first produced in 1963 and improved upon numerous times over the past 40 years, is currently slated for a comprehensive overhaul. The M109 PIM upgrade shares many common components of the Bradley Fighting Vehicle, allowing for more commonality with other Army vehicle systems and maximizing production savings, parts inventory and maintenance personnel while avoiding component obsolescence.

The PIM modernization effort is a significant upgrade of the M109A6 Paladin which includes buying back Space, Weight, and Power - Cooling. While the self-propelled howitzer's cannon will remain unchanged, the PIM will sport a brand new chassis, engine, transmission, suspension, steering system, to go along with an upgraded electric ramming system.